Note to teachersThe overall goal in this exhibit is to give students and teachers a glimpse into a moment of discovery. The discovery of opticalpulsars may be the only example of a significant discovery documented by a tape recorder left running for other purposes. The listeneris privileged to hear an event as it happened, not the staging of an event.

This exhibit allows students and teachers to recognize scientists as people. The thrill of discovery provides a human element towhich everyone can relate. As Cocke and Disney check their results and share their excitement, they are people engrossed in science,not humorless, unfeeling machines recording data.

This exhibit also provides a compact view of the scientific process in operation. Experimental science is presented in a truecontext. The astrophysicists are constantly observing, manipulating equipment, and then repeating the observation in order to becertain of their results. Mathematics, data and instruments are all checked. Only after they have tried every way they can think of tomake their discovery "go away," and still find it staring at them, are the scientists satisfied.

This exhibit can also be an opportunity for professional development. Science teachers can strengthen their background in pulsarsand neutron stars -- one of the most fascinating new fields of astronomy -- through self-study of the module and linked Web sites.Teachers can better understand the struggle of scientists to understand the nature of this interaction as they listen to the scientiststhemselves describe their involvement.

In an astronomy course: The unit can be used during a discussion of stellar evolution or at any other point where the topic of pulsars isspecifically addressed.

In a physics course: The unit can be used in connection with discussions of gravity, density of matter, conservation of angularmomentum, or especially when discussing the different forms of electromagnetic radiation. It may also be used as a brief excursioninto scientific discovery or scientific method.

In a history or philosophy of science course: This unit can be used as a true account of one type of discovery in science.

We need your feedback so we can do more exhibits like this! Both our funding and our enthusiasm could falter if we don't hearfrom users. Please e-mail us at chp@aip.org or use the online form (at https://webster.aip.org/forms/feedback.htm) to tell us howuseful this was to you (a brief word is great, comments and suggestions better still).

Contents of This UnitAudio clips and accompanying text: These are central to every format of presentation. Test trials of these materials showed that inclassroom use, it is best to have students read the script simultaneously with listening to the audio, rather than listen to the audio alone.Permission is granted to the instructor to make photocopies of the script for the purpose of providing every student or every pair ofstudents with a script, for classroom use.

The exhibit can conveniently be broken into two segments. The first part includes narration and excerpts from interviews with Cockeand Disney relating the events leading up to the discovery. The second part is the audio recording of the scientists on the night of thediscovery.

We need your feedback so we can do more exhibits like this! Both our funding and our enthusiasm could falter if we don't hearfrom users. Please e-mail us at chp@aip.org or use the online form (at https://webster.aip.org/forms/feedback.htm) to tell us howuseful this was to you (a brief word is great, comments and suggestions better still).

Articles:

The unit includes original research papers excerpts—Hewish, Bell, et al., "Observation of a Rapidly Pulsating Radio Source," Nature vol 217, p.709 (24 Feb. 1968).

Cocke, Disney, and Taylor, "Discovery of Optical Signals from Pulsar NP 0532," Nature vol. 221, p. 525-27 (8 Feb. 1969).

a primary resource—extract from laboratory notes of John Cocke, night of 15 Jan. 1969.

articles on the physics of pulsars—DiLavore and Wayland, "Pulsars for the Beginner," The Physics Teacher vol., p.232-237 (May 1971).

media account—"Pulsar Detected Optically," Sky and Telescope, p. 135 (March 1969).

and additional readings—Table of contents from Davies and Smith, ed., The Crab Nebula, International Astronomical Union Symposium no. 46, Aug.5-7, 1970(Dordrecht: S. Reidel; N.Y.: Springer Verlag, 1971).

Application for time on a telescope (1983).

Exercises: The unit presents suggestions for assorted activities, demonstrations, questions, problems, and experiments.

These exercises have been labeled as Discussion, Investigation, or Research.

—Discussion exercises (D) require no preparation or reading by the student. These exercises can be used for class discussions or ashomework assignments.

—Investigation questions (I) require the reading of an article which is included in this unit, or the use of reference works such as anencyclopedia. Instructors can make the articles available for a more comprehensive assignment.

—Research questions (R) require library work. Some of these exercises are quite extensive and should be treated as long-termprojects.

The physics/astronomy exercises are identified as simple or complex.

—A simple exercise (S) requires no background material and is a suitable class or homework assignment.

—A complex exercise (C) requires that the student have access to a physics text or to some laborabory equipment.

Bibliography: The unit includes an annotated bibliography for instructor and student use.

The National Standards

The National Science Education Content Standards "outline what students should know, understand and be able to do in the naturalsciences over the course of K-12 education." Although most science teachers are aware of the subject matter understandings (i.e.,Physical Science Standards) in their respective disciplines, too little attention is devoted to the categories of:

• Science in personal and social perspectives

• History and nature of science• Science and technology• Science as inquiry

This exhibit provides material that speaks to these dimensions of science content knowledge as well as the required Physical ScienceStandards. This exhibit is an excellent vehicle by which to bring the full Content Standards to the science classroom.

Physical Science and Earth & Space Science Standards: The Pulsar Discovery includes some subject matter understandingsfrom both the 9-12 Physical Science and Earth & Space Science Standards. The size of the pulsar leads to exercises concerningrotation rates and nuclear forces and magnetic fields. The decrease in the rate of spinning provides an example of energy conservation.The exhibit also provides insight into stellar evolution.

Science in personal and social perspectives: The investigation of pulsars appears not to have direct impact on the health orwell-being of our society. Students should have an opportunity to discuss some of the issues which policy makers must address:Should such investigations be supported with tax dollars? Who makes decisions on which proposals will be funded and at what cost?Should research with pre-determined applications be the only ones that are funded? Should pulsars only be studied if they can producea better clock?

History and nature of science: The Pulsar Discovery unit and related teachers' guide provide an example of curriculummaterials that support this content standard. The script and exercises emphasize Science as a Human Endeavor, speak to the Nature ofScientific Knowledge and provide Historical Perspectives. A student gets a rare glimpse into the creation of new knowledge as a real-time tape provides an account of scientists with their guard down.

Science and technology: The scientists involved in the discovery of the pulsar note the interplay of science and technology invarious discussions. Cocke and Disney needed the "ideal piece of equipment … belonging to Don Taylor." In contrast to thesophisticated equipment, they had to build for themselves a tiny diaphragm out of a piece of aluminum foil.

Science as inquiry: One component of the inquiry content standard is that students should understand that scientists engage ininquiry and the nature of that engagement. In the Pulsar Discovery, students learn about the types of questions that scientists ask, howthey rely on technology to gather data, how mathematics is used and how scientific explanations must adhere to specified criteria.They are also introduced to different kinds of investigations and communications of scientists. Students get to examine the laboratorynotebook as a primary source illustration of the artifacts of science as inquiry.

Lesson Plans

The instructor can allot no class time:The students can benefit from the audio through independent study. Students visit the exhibit online and perform the exercisesassigned by the teacher. For example, one exercise can be chosen and given to the entire class. Or different exercises can be assignedby the first letter of the student's last name. Or, as a third alternative, a group of exercises can be offered and each student can choosewhich avenue to explore.

The instructor can allot one class day:—Students can visit the exhibit as a group during class time. In this case, scripts should be available to every student or pair ofstudents. Permission is granted to the instructor to make photocopies of the text for the purpose of providing every student or pair ofstudents with a copy for classroom use. Students may be given one homework assignment prior to listening to the audio and one afterlistening to the audio.

The uninterrupted audio track and all the rest of the unit can be purchased at nominal cost on a CD-ROM by filling out the order formonline at http://www.aip.org/history/mod/order.html which can be played in class or made available to individual students who haveaccess to a computer.

MOST TEACHERS WILL PROBABLY FIND THE ABOVE THE BEST WAY TO USE THE UNIT.

OR—Teachers may elect to have students view the exhibit independently (see above) and later use class time for discussion andexercises.

The instructor can allot two class days:

DAY 1: Students view the first half of the exhibit.The remainder of the class is used for comments on one or more discussion questions, such as those in the accompanying list.

A homework assignment should be chosen, for example from the accompanying list, reflecting teacher or student interest.

DAY 2: Students view the second half of the exhibit.

The beginning of the class may center on reviewing homework or on fresh questions chosen by the instructor. The remainder of theclass time may be spent on discussion questions emphasizing the exhibit as a whole.

Homework should be assigned. This might require the reading of an article included in the unit, or library research.

The 5E Model

The 5E model of good science instruction recommends that teachers structure the lesson so that it includes the following components:engage, explore, explain, elaborate and evaluate.

In using the Pulsar Discovery exhibit, teachers can adopt the 5E model in the following manner:

Engage: Students should be questioned about the size of astronomical objects like the Moon, Earth, Sun and other stars. Students atthe high school level can still be expected to harbor misconceptions concerning the rotation and revolution of the Earth, no knowledgeof the rotation of the Sun, and little knowledge of how the solar system is held together. The questions of scale can engage studentsand provide a basis for a better appreciation of the pulsar. Without such a preliminary discussion, students will not understand why thediscovery of the pulsar came as such a shock to scientists the world over. Students should be given the opportunity to articulate theirprior conceptions. Teachers should be attentive to the students' understanding so that the subsequent instruction can provide a rationalefor students to continue their prior beliefs or to replace them based on their study.

Explore: Students can read and listen to the script and begin to explore the events leading to the discovery of the optical pulsar. Theycan continue their exploration by responding to some of the exercises including the brief laboratory activities on the use of a manualstroboscope and the creation of a diaphragm. They can also review the notes that Disney recorded on the night of the discovery.

Explain: Students should study the articles that are included in the exhibit. The original research articles may be a bit difficult in theirentirety but should be attempted. Science students get too few opportunities to read any original literature. The other articles andessays were chosen as part of the exhibit because of the different perspectives that they bring to our understanding of pulsars.

Elaborate: Students should have the opportunity to apply the knowledge from the script to new situations. Students should review theTable of Contents from a Crab Nebula symposium held just a few years after the discovery of the first pulsar. They can completeexercises related to the physics of pulsars as well as exercises related to the tape recording, the interaction of the scientists, and thescientific processes that were used to insure that a mistake had not been made.

Evaluate: Many of the exercises can be used as evaluative tools of what students understand and are able to do. The teacher shouldhelp students set the criteria for successful achievement. What is the level of expectation in terms of the physics problem solving orthe related research items? Evaluations can also include group projects that require students to produce informational pamphlets, toperform or create additional physics simulations, or to compose an essay or play that draws out the human and scientific elements inthe pulsar discovery.

The 4 Question Model

The 4 Question model of science instruction requires that students be able to answer the following questions:

• What does it mean?• How do we know?• Why do we believe?• Why should I care?

In using the Pulsar Discovery exhibit, teachers can adopt the 4 Question model in the following manner:

What does it mean? Students should be able to provide a sense of the size of the pulsar and the rotation rates of the pulsar. Theyshould also be able to explain the decrease in the rotation rate using conservation laws. They should also be able to explain the contentstandards of the National Standards in the domains of inquiry, technology, society and history.

How do we know? We know because we made observations. How were Cocke and Disney able to measure the rotation rate of thepulsar? How were they sure that their data were accurate? Why was it important to know the expected period of 33.2 millisecondsbefore beginning the experiment? How would the experiment have changed if they did not know the period from the outset?

Why do we believe? Why do we believe that the pulses are naturally occurring and not the result of an extraterrestrial being trying tocontact us? (The first pulsar was initially suspected to be such contact.) The size of the pulsar demands that the origin be a neutronstar. The models for stellar evolution must allow for the incredible compression of matter. Calculations of forces, stellar evolution,conservation laws and a mechanism for explaining our observations must all hold together if we are to believe in the optical pulsarresults.

Why should I care? The pulsar has very little relevance to our lives. Similarly, the electrical investigations of Faraday, Ampere,Maxwell and Hertz had little relevance at that time. If scientists like those mentioned had been required to work on improving modesof communication, the discovery of radio waves and the electromagnetic spectrum would probably not have occurred. Their interestsand investigations led to the development of radio, television, and the transistor (http://www.pbs.org/transistor/) along with all of theaccompanying social changes. There are similar examples of pure research into the behavior of atoms leading to discoveries that foundapplications in MRI and other medical technologies. Who should decide what research should be funded? Why should we care aboutpure scientific research?

A WORD TO THE WISEPhysics teachers may well lack experience in leading discussions. We all know, however, that it is not possible nowadays to thinkabout science without taking account of different viewpoints on social questions. Since the interaction of science and society is oftentaught only superficially in social studies courses, science teachers need to explore the issues. It is recommended that teachers have anumber of discussion questions created or chosen from those provided, so that if one does not develop into a useful class dialogue, asecond or third question can be presented.

History teachers frequently lack experience with science demonstrations, problem solving, or explanation of scientific theories. We allknow, however, that the citizen can no longer separate understanding of modern history from basic ideas about science itself. Sincemany students have a very rudimentary science education, a unit in the history of science may be one of their few encounters withscientific reasoning, and it is important to be sure they can follow the logic of the science itself. It is recommended that the instructortest a demonstration before presenting it to the class. Similarly for science problems, a previously worked out solution or explanationwill always lead to a better class presentation.

We need your feedback so we can do more exhibits like this! Both our funding and our enthusiasm could falter if we don't hearfrom users. Please e-mail us at chp@aip.org or use the online form (at https://webster.aip.org/forms/feedback.htm) to tell us howuseful this was to you (a brief word is great, comments and suggestions better still).

Suggested Exercises

1. The maximum possible size of a pulsar can be roughly determined from the fine structure of thepulses emitted, reasoning as follows.

If the sun were to turn off all over instantaneously, we would not see the light cease instantaneously,for the last light from the near side of the sun would reach the earth a little sooner than the last lightfrom the far edge of the sun. Given that the sun's radius is 6 X 108 meters, how much time wouldelapse between the arrival of the two bits of light?

A flash from a pulsar may have a fine structure with typical time of rise and fall around 30i d U thi t t h ti t f th i ibl i f h t i itti

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the flash.

2.Pulsars are thought to be rotating neutron stars. The pulses probably emanate from the magneticpoles, which rotate with the star (as with the earth, these poles may not be aligned with the axis ofrotation). The light goes straight out from the poles, like a beam from a searchlight, and on everyrotation, the beam may sweep across the earth.

What would we see if the magnetic poles of the pulsar were exactly aligned with its axis of rotation,and pointing toward the earth?

From this "lighthouse" model, would you conclude that there are many more pulsars than we haveobserved? Why? Construct an equation for the number of unobserved nearby pulsars, in terms of thenumber of nearby pulsars that have been observed. (Before writing the equation, you will need todefine some symbols in terms of the geometry of a typical pulsar beam.)

S

3.The energy of a pulsar's emission is drawn from the pulsar's rotation, resulting in a slowing down ofthe rotation rate. This slowing down has been measured. The Crab pulsar, which has a rotation periodof 33 milliseconds, is slowing at the rate of 4.2 X 10-13 seconds per second.

What is the rate of slowing in seconds per year? If you used this pulsar as a clock, how much timewould pass before you were "slow" by one minute?

The age of a pulsar is given approximately by the equation

where T1 is the period of rotation and 1/T is the rate of change in the period. Using this equation, howold is the Crab pulsar?

S

4. LABORATORY EXERCISE: measuring short time intervals using stroboscope.

The principle of the stroboscope can be demonstrated with a slowly blinking light, such as a spotlighton the street. If you blink your eyes at the same rate as the light, you can make it seem to yourself as ifthe light is always off, by having your eyes closed whenever the light is on. Or you can make it seem asif the light is always on. By controlling the way you look, you can make something that moves in arepetitive way "stand still." This is precisely like the way Taylor's electronics helped Cocke and Disneymake the pulsar's blinking "stand still" so they could see it.

The hand stroboscope lets you do this for things that move more rapidly than the eye can blink. It is adisk with several equally spaced slits. As the disk is rotated in front of the eye, the eye catches aglimpse of the moving object, which can appear to be stopped. For example, you may look at a rotatingcircular saw with a red spot painted at one position. If you make the spot seem to stop by looking at thesaw through a hand stroboscope of 6 slits, turning at 35 revolutions in 5 seconds, you can calculate: .,

therefore the saw is rotating at 42 revolutions per second.

(a) Could the saw also be rotating at 84 revolutions per second? at 21 revolutions per second?

(b) If the spot on the circular saw appears to move slowly backward when seen by a stroboscope, is thestroboscope rotating a little faster or a little slower than the rate that will hold the spot still? Explain.(This is precisely similar to Cocke and Disney's concern with holding the pulse in the middle of theirscreen.)

(c) A television camera looked at the Crab pulsar (whose period is .033 seconds) through astroboscope. If the strobe had 8 slits, at what rate should it rotate to make the pulsar seem always litup?

(d) Use a hand stroboscope to measure the rotation rate of an electric fan; the vibration rate of a bell.

(e) What do you see if you look at a fluorescent light through a stroboscope? (The effect cansometimes be seen by looking at a rotating object, such as an electric fan or the gears on a rotary handdrill, under a fluorescent light.)

C

5. Much as students make applications for college, astronomers must make applications to use majortelescopes. Read the "Application for Time on a Telescope" form used by a large observatory.Comment on each portion of the form. Specifically, why does the committee that decides who getstelescope time need an answer to each question? Imagine that you are Disney or Cocke and fill out theapplication in order to convince the committee to give you time to search for an optical pulsar.

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6. LABORATORY EXERCISE: Make a diaphragm. Cocke and Disney described a diaphragm thatthey needed to observe the pulsar (so their light-detecting device would not be swamped by the light ofother nearby stars). They made it with a razor and aluminum foil.

(a) Comment on this aspect of a modern experiment. Should they have used more refined apparatus?How would their story have been different if they had required a special diaphragm that took a team oftechnicians a month to build?

(b) Make a diaphragm ten times the size of theirs: a square hole in foil exactly 1 mm x 1 mm. Then tryfor one 0.1 mm x 0.1 mm.

(c) Can you think of a better way for making such a diaphragm at home? (One possibility: coat a pieceof glass with paint, let it dry, and scratch some of the paint away.) Try other methods.

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7. R. Willstrop of Cambridge, England, was taking data from the Crab pulsar months before Cocke andDisney. However, Willstrop needed a large amount of computer analysis of his data, so he was not ableto announce the "discovery" of an optical pulsar until after Cocke and Disney. Who should get creditfor the discovery? What difference does it make who gets the credit? Some sociologists believe thatcredit for a discovery is the chief reward a scientist strives after. How important to progress in sciencewas assignment of credit in the pulsar story?

A student, Jocelyn Bell, was first to observe a radio pulsar while analyzing data—she noticedsomething peculiar that nobody else had paid attention to, and hunted down its source. But a NobelPrize was awarded to Anthony Hewish, who had assigned her the task and who had built the radiotelescope. There has been some controversy over whether the prize award was fair. Compare this withthe assignment of credit in the optical pulsar story. How much credit should go to the builder of aninstrument, like Hewish or Taylor? How much to the person who designs a research program, likeHewish, and how much to the person who carries it out in a creative way, like Bell? Does it matter?

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8. Consider the following circumstances of the optical pulsar discovery: (a) Cocke and Disney meet;(b) Taylor has a suitable apparatus; (c) a new fast pulsar has recently been discovered (it turns out thatof the hundreds of pulsars observed with radio telescopes, only a handful are readily seen with ordinarylight); (d) they get extra observing nights because a colleague's wife is ill; (e) they find a math errorwhen they recompute their values; (f) they hold PhDs, representing years of hard training in science.

How much luck was involved in the discovery? How much hard work? In your answer, refer to theabove circumstances as well as any others that seem relevent.

A favorite motto of scientists was stated by Louis Pasteur: "In the realm of observation, chance favorsthe prepared mind." In what way does or does not this saying apply to the pulsar story?

D

9. Optical pulsars help us understand neutron stars, a late stage in the life cycle of some stars. What isthe significance of Cocke and Disney's work? If we understood the life cycle of stars better—so what?Should research of this type be supported with tax dollars?

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10. When Taylor got the excited phone call from Disney and Cocke, he was skeptical at first. Wouldmost scientists have felt that way? How important is skepticism in science? How skeptical should yoube when you hear something from (a) a textbook; (b) a famous scientist on television; (c) a scientist ontelevision identified as a member of a "public interest" group; (d) a television reporter? Would a goodscientist be more or less skeptical than you of each of these sources? How do scientists finally decidewhether what they have been told is true?

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11. Compare the audio recording of the discovery, the paper describing the results in Nature, and (ifyou can find one) a newspaper article announcing the discovery. Which comes closest to describingwhat was in fact discovered? Which is most objective? Is anything entirely objective?

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12. The official communication of the discovery in Nature is written in a highly stylized form. Why isthis form used?

Take a long, hard look at some everyday phenomenon (for example, water emptying from a sink, or asunset) and imagine that it has never been observed before. Write (a) a paper in scientific style, and (b)a letter to a friend about your "discovery." What purposes are served by each style?

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13. The audio recording that you have listened to was pieced together from interviews with threepeople and from the tape made at the time of the discovery. In what ways is the tape made at the timelikely to be more truthful than the accounts recorded a decade later? In what ways might the tape madeat the time be less useful to understanding what happened? Some historians think it is a waste of timeto interview people about what they did many years ago; these historians spend their time studying oldcorrespondence and other writings recorded near the time of the events they study. Other historians tryto interview as much as possible. Discuss the advantages and disadvantages of each approach. Usesome examples from the optical pulsar story.

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14. If you could stand on a neutron star, how far away would the horizon appear? C15. Name some objects that rotate at about the same speed as a pulsar. Which have the most stable rateof rotation? Why? C16. Review the description of the environment as Disney saw it on his way to the observatory. Is thisyour impression of what a scientist would notice? Is it different from what a non-scientist wouldnotice?

D

17. We know that ice skaters can spin faster by pulling their arms in: the angular momentum isconstant, but the decreased radius produces an increased rotation rate. Calculate the angular momentumof our sun (radius 6 X 108 meters, period of rotation 25.3 days), making an arbitrary assumption aboutthe distribution of mass. Using conservation of angular momentum, calculate the new rotation rate ifthe sun shrank to the size of a neutron star (radius 10 kilometers = 104 meters). How does this comparewith the rotation rates of pulsars?

C

18. If all the mass of the sun were to become a neutron star, what would be its density? (Solar mass = 2X 1030 kilograms.) How does this compare with the density of the nucleus of an atom? If you could geta piece of a neutron star the size of a pea, how much would it weigh on earth?

C

19. On the earth, the gravitational force at the equator is partly balanced by a centrifugal force whichtends to throw things off this spinning planet (that is why the earth bulges slightly at the equator). Findthe gravitational and centrifugal forces at the equator of a pulsar 10 km in diameter with a pulseinterval of 33 milliseconds. Compare these with the same forces on earth. Would you expect much ofan equatorial bulge on a pulsar?

C

20. Calculate an example of Coriolis forces on the surface of a neutron star, assuming a plausibleposition and velocity for a moving object. If creatures somehow lived on the surface of the neutronstar, would Coriolis forces severely affect their movements?

C

21. The sun has a magnetic field at the surface averaging about 1 gauss. If the sun were to shrink to thesize of a neutron star, the field lines would not escape but would become more tightly wrapped. Whatmagnetic field strength would the neutron star have? Does your result seem reasonable? If creatureslived on the surface of the neutron star, and if their bodies had magnetic properties like our own, wouldthey have to take magnetic field lines into account in their movements?

(Note for the teacher: There is a science-fiction novel by the astrophysicist Robert L. Forward,Dragon's Egg, Ballantine: 1980, which imagines life on a neutron star. The "biochemistry" is made ofcombinations of nuclei, which can exist in a very shallow surface zone. The creatures are blobs a fewmm across. While gravity is too great for them to throw objects, anything they roll is noticeablyaffected by Coriolis forces. Magnetic field lines are far more important: at the magnetic equator, acreature is pulled out into a cigar shape; it can move along field lines easily, but across them only withgreat difficulty.)

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22. Find the Crab Nebula on a star map. Can you find it in the sky during XXX? (Note for the teacher:fill in month when exercise is assigned.) I23. In November 1982 a new pulsar was discovered, PSR 1937 + 214, better known as "the millisecondpulsar." Its period is 1.56 ms, that is, a frequency of 642 pulses per second, twenty times the rate of theCrab pulsar. Calculate the gravitational and centrifugal forces for this pulsar, assuming a diameter of10 km. What is the velocity at the equator? Is this close enough to the speed of light so that usefulcalculations would have to call upon the equations of special relativity?

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24. The "South Preceding" star in the Crab Nebula (the Crab pulsar) was recorded on a photographicplate made in 1899, and preserved since then. State some questions that could be answered bycomparing this old plate with a recent one, and that could not be answered by a recent plate alone.

Old astronomical photographic plates are worth preserving for scientific use, whereas old laboratorymeasurements in physics are of little scientific value—because a physicist can always measure aphysical quantity anew, and usually more precisely than the first time. Astronomy, but not physics, isin part a "historical science." Name some other "historical sciences." How are they like the study ofhuman history? How are they unlike it?

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25. Following the audio part of the exhibit, outline the steps in the process of the optical pulsarobservation. Include "null" observations, checks of various kinds, changes in the data accumulated,repetitions of earlier trials. From this outline and your general ideas about scientific process, answer thefollowing:

(a) How skeptical are Disney and Cocke?

(b) Why does Taylor tell them to sit on their results?

(c) Does this case agree with your ideas about "good" scientific method? Explain your answer.

(d) Examine the sample from Cocke's notebook. Does this agree with your ideas about how "good"scientists record data?

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26. Inspect the page from the proceedings of a pulsar conference. Using this as evidence, estimateabout how many people were working on pulsars at that time? State your reasoning.

Some scientific subjects are studied only occasionally by people who soon move on to other subjects.Other subjects may become institutionalized fields, first with conferences, then perhaps with textbooksand university professorships, finally with entire journals and scientific societies dedicated to the field.How far along this series of steps would you expect the study of pulsars to go, and why? Name somethings that might influence whether a subject of inquiry will attract the full-time attention of manyhundreds of researchers?

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27. Define the Doppler effect in general. What effect did it have on the search for an optical pulsar? R28. Suppose a pulsar is about equally hard to detect at radio and at optical light frequencies.Atmospheric absorption of radio and optical frequencies is slight, and instruments to detect theirenergy are of roughly equal sensitivity. A typical major radio telescope has 50 times the diameter of amajor optical telescope.

(a) Roughly what is the ratio of the energy the pulsar emits in radio frequencies to the energy it emitsin optical frequencies?

(b) Suppose the pulsar emits the same amount of energy in X-ray frequencies as in optical frequencies,should this be very easy or very hard to detect? Give two reasons for your answer.(Note for the teacher: X-rays from pulsars have been detected by satellites above the atmosphere.)

(c) Why can a radio telescope be built much larger than an optical one? What sets the limit on how biga radio telescope can be?(Note for the teacher: The largest radio telescope is at Arecibo, Puerto Rico, and fills a naturalhemispherical depression 305 meters in diameter.)

A larger project for an advanced student or student team can be made from four Web exhibits whichall describe "moments of discovery:" nuclear fission, an optical pulsar, the electron, and the transistor.The student(s) should study all four exhibits and discuss similarities and differences -- socially in termsof individuals, scientific institutions, and communication, and scientifically in terms of technologiesand thought processes. Students can review the lists of questions in the Teachers' Guides for ideas ondirections to follow (perhaps too many!). The students should conclude with general statements aboutthings that seem necessary for all discoveries, at least in modern physical science.

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Additional Reading and LinksUnless otherwise noted, the level is appropriate for middle-school students and above. Popular books and textbooks are beingpublished at such frequency that any bibliography is quickly out of date. The references that are listed here serve as a general view ofwhat is usually available..

Web Sites

Neutron Stars and Pulsarshttp://imagine.gsfc.nasa.gov/docs/science/know_l1/pulsars.htmlBrief information from NASA's Goddard Space Flight Center, with resources and a "Teachers' Corner".

The Sounds of Pulsarshttp://www.jb.man.ac.uk/~pulsar/Education/Sounds/sounds.html

Sound clips give a physical feeling for the rapid spinning of these incredible objects.

Astronomy, Astronomers, and the Steward Observatoryhttp://www.as.arizona.edu/steward/Good general information on astronomy and astronomers, plus what's up at the University of Arizona's Steward Observatory today:telescopes, people, research.

Astronomy at Different Wavelengthshttp://www.ipac.caltech.edu/Outreach/Multiwave/From Caltech, a readable explanation of how astronomers make use of many kinds of telescopes to study objects in wavelengths fromradio to x-rays.

The Princeton Pulsar Grouphttp://pulsar.princeton.edu/pulsar/multimedia.shtmlThe group offers some basic pulsar information and visual and sound clips for the public. The site also lets you see how they carry ontheir work (scientific papers, telescope schedules, etc.).

Jodrell Bank Observatory Pulsar Grouphttp://www.jb.man.ac.uk/~pulsar/Another major research group in action, including college-level course materials complete with technical equations and a bit ofhistory, and more links.

NASA Space Linkhttp://spacelink.msfc.nasa.gov/An educators' gateway to NASA's huge resources, mostly on spaceflight and the planets but including astronomy.

NASA's Astronomy Picture of the Dayhttp://antwrp.gsfc.nasa.gov/apod/Fascinating browsing on all astronomical topics.

Readings

Levy, David H. Skywatching (A Nature Company Guide). New York:Time-Life Books, 1995.

Bite-size chunks of information on astronomy including historical overview, astronomers today, skywatching guide with constellationsetc., and descriptions of astronomical objects including pulsars.

DeVorkin, David, ed. Beyond Earth : Mapping the Universe. Washington, DC: Smithsonian Air & Space Museum and NationalGeographic Society, 2002.

A lavishly illustrated history of cosmology from ancient times to the present, particularly rich in information about instruments. By aleading historian of modern astronomy.

Ferris, Timothy. The Whole Shebang: A State of the Universe(s). New York: Simon & Schuster, 1997.

Little on pulsars, but this is among the best-written of the popular-level descriptions of the history and status of modern cosmology, bya premier science journalist. (Note however that the field advances so quickly that within a few years all books get partly out of date.)

Kaufmann, William J. III, and Roger Freedman. Universe. New York: W.H. Freeman, 5th ed., 1998.

Pasachoff, Jay. Astronomy: From the Earth to the Universe. Pacific Grove, CA: Brooks/Cole, 6th ed., 2002.

These are two of a number of readable and well-illustrated textbooks designed for introductory college astronomy courses, andaccessible to an advanced high school student with a strong interest in science. Pulsars are discussed in chap. 23 ofKaufmann/Freedman and chapter 30 of Pasachoff.

Lyne, Andrew G., and Francis Graham-Smith. Pulsar Astronomy (Cambridge Astrophysics Series, Vol 16). Cambridge:Cambridge University Press, 2nd ed., 1998

An authoritative technical work for advanced science students and practicing researchers, with a chapter on the history of pulsardiscovery.

MAGAZINES: To keep abreast of the latest discoveries in astronomy, consult recent issues of Sky and Telescope and Science News,both available in most libraries.

Reprinted ArticlesGeorge Greenstein, excerpt from Frozen Star, New American Library Trade, p.13-31 (May 1985).

Hewish, Bell, et al., "Observation of a Rapidly Pulsating Radio Source," Nature vol 217, p.709 (24 Feb. 1968).

"Discovery of Optical Signals from Pulsar NP 0532". W.J. Cocke, M.J. Disney, and D.J. Taylor. Nature, February 8, 1969,volume 221, p. 525.

John Cocke's notes on the observations made with Michael Disney at Steward Observatory on the night of January 15, 1969.

"Pulsars for the Beginner". Phillip DiLavore and James R. Wayland. The Physics Teacher, May 1971, p. 232.

"A Pulsar Detected Optically" Sky and Telescope, p. 135 (March 1969).

Table of contents from David and Smith, ed., The Crab Nebula,Inernational Astronomical Union Symposium no. 46, Aug. 5-7, 1970.

Kitt Peak National Observatory Observing Time Request.

About This ExhibitThis exhibit is based on the educational package, Moments of Discovery, Unit 2: A Pulsar Discovery, by Arthur Eisenkraft withLilllian Hoddeson, Joan N. Warnow, Spencer Weart, and Charles Weiner, published by the American Institute of Physics in 1984.Copyright ©1984 and 2003 American Institute of Physics(http://www.aip.org/copyright.html)Exhibit Editors: Spencer Weart (sweart@aip.org)

Patrick McCray (pmccray@aip.org)Arthur Eisenkraft (eisenkraft@worldnet.att.net)

Design: Linda Wooliever (info@imaginecreativity.com)

For further information

Further information on the history of physics can be obtained from the AlP's Center for History of Physics(http://www.aip.org/history/) in College Park, MD.

We need your feedback so we can do more exhibits like this! Both our funding and our enthusiasm could falter if we don't hearfrom users. Please e-mail us at chp@aip.org or use the online form (at https://webster.aip.org/forms/feedback.htm) to tell us howuseful this was to you (a brief word is great, comments and suggestions better still).

Photo and Voice Credits

PHOTO CREDITS:

Page 1

Picture of Morrison. AIP Emilio Segrè Visual Archives.

Page 2

Telescope. Credit: Photograph by Jim Scotti.

Picture of Disney, Cocke, and Taylor at Steward Observatory , 1969. Photograph by Robert M. Broder. Copyright ©Time PixSyndication.

Page 3

Expanding Crab Nebula. Credit: Adam Block (KPNO Visitor Program), NOAO, NSF.

Page 4

Telescopes in winter. Photograph by Jim Scotti.

Page 5

Page 6

X-ray image. Credit: Chandra X-ray Observatory, NASA.

Page 7

Page 8

Crab image. Credit: J. Hester and P. Scowen (ASU), NASA

Page 9

Crab image. Credit: FORS Team, ESO.

Page 10

Crab in gamma ray. Credit: NASA, Compton Gamma Ray Observatory.

VOICE CREDITS:

Excepts from interviews for this exhibit with Jon Cocke, by Joan Warnow and Spencer Weart in February 1975; with Michael Disney,by Inge Disney in February 1975; and with Philip Morrison by Joan Warnow in July 1975.

2003 American Institute of Physics